The present invention relates generally to a land-mobile radio communication system, and in particular to a land-mobile radio communication system employing a number of fixed sites, a number of mobile stations and a number of communication channels.
Land-mobile radio communication systems provide a vital service for many types of operations, such as police and fire departments. One embodiment of such a system is a digital system, for data or voice, employing a number of fixed sites separated geographically. Mobile stations generally communicate with the nearest fixed site, and in so doing, a large geographical area can be served without unreasonably high transmitter powers or undesirably high antenna towers. Often, different channel frequencies are used at the various sites to avoid interference and increase throughput. If the number of frequencies available is less then the number of sites required to cover the desired area, a frequency reuse plan may be employed. Up to now, the channels at the various sites have the same bandwidth and the bandwidths are fixed.
Due to the recent increase in demand for data communications services, such as Internet services like the World Wide Web, the size of data messages and their frequency of transmissions have increased and take longer to transfer on existing land-mobile radio communication systems. New cellular radio systems, such as TDMA systems, like GSM, or CDMA systems, like UMTS, allow the data rate available to an individual mobile station to be changed on demand. This is done in TDMA systems by allocating a larger number of time division slots per unit time to the particular user requiring greater throughput. In CDMA systems, a greater number of spreading codes are assigned to the user requiring greater throughput. In both of these systems, wideband channels are subdivided in time or code domains into smaller xe2x80x9clogicalxe2x80x9d channels and a control channel is used to inform mobile station of the time slots or spreading codes to be used by the mobile stations. To use these mechanisms, the system operator must have at least one large channel (e.g., 200 kHz for GSM and 5 MHz for UMTS), and to cover a large area, GSM systems require additional channels. But some operators, like public safety operators, have only a few narrowband channels allocated by law in a spectrum segment that does not support GSM or UMTS.
Thus, there exists a need for a land-mobile radio communication system that can provide high data rates when needed, but that does not require more radio bandwidth than the user already has.